feat(notification-outbox): register NotificationOutbox singleton in Host

Wire the Notification Outbox into the Host central role:
- Program.cs: call AddNotificationOutbox() on the central path (binds
  NotificationOutboxOptions via BindConfiguration; no explicit Configure).
- AkkaHostedService.RegisterCentralActors(): create the NotificationOutboxActor
  as a non-role-scoped central cluster singleton + proxy, then send
  RegisterNotificationOutbox(proxy) to the CentralCommunicationActor.
- appsettings.Central.json: add the ScadaLink:NotificationOutbox section with
  the NotificationOutboxOptions defaults.
- SiteServiceRegistration: remove the now-dead AddNotificationService() call -
  sites forward notifications to central rather than delivering over SMTP, and
  no site component consumes the SMTP machinery.
- Host.csproj: add the ScadaLink.NotificationOutbox project reference.
- Tests: add central outbox singleton/proxy actor-path assertions, drop the
  site OAuth2TokenService/INotificationDeliveryService resolution assertions,
  and add NotificationOutbox to the component-library IConfiguration check.

tests/ScadaLink.Host.Tests/ActorPathTests.cs

@@ -97,6 +97,14 @@ public class CentralActorPathTests : IAsyncLifetime
     public async Task CentralActors_Management_Exists()
         => await AssertActorExists("/user/management");
 
+    [Fact]
+    public async Task CentralActors_NotificationOutboxSingleton_Exists()
+        => await AssertActorExists("/user/notification-outbox-singleton");
+
+    [Fact]
+    public async Task CentralActors_NotificationOutboxProxy_Exists()
+        => await AssertActorExists("/user/notification-outbox-proxy");
+
     private async Task AssertActorExists(string path)
     {
         Assert.NotNull(_actorSystem);